It works for all attibutes whose methods defined in "ISubject", but the methods for "extendedAttribute" are defined in IExtendedAttributeAware so i got the message:
[EL Warning]: 2012-01-05 18:30:02.308--Ignoring attribute [extendedAttribute] on class [de.unituebingen.medizin.uak.ergexplorer.interfaces.ISubject] as no Property was generated for it.

I use "xml-transient" attribute like at this page:

www.eclipse.org/forums/index.php/t/205662/

I tested it with classes and it works fine, but it looks like i cant use it with interfaces.